1. Field of the Invention
The present invention relates generally to electrical switching systems, and is specifically concerned with a high speed transfer switch that is capable of transferring a multiphase load between different multiphase A.C. power sources on a phase-by-phase basis in such a manner as to insure a stable and continuous delivery of power to the load.
2. Description of the Prior Art
Brief electrical outages in a power distribution system frequently cause long lasting impact to the equipment fed by the system. These outages may only last a fraction of a second in duration, are often sporadic in nature, and can occur without warning. Despite their brevity, such power outages can be sufficient to cause data processing systems and other sensitive equipment to lose memory and other stored information. Disruption of internal memory may cause these systems to stop and reload their control programs, or they may simply cease to operate altogether. The uncoordinated operation of these machines may be life threatening and may place significant amounts of property and money at risk. The loss of control in a hospital, automotive assembly plant, or banking institution computer are but a few of the many possible examples.
Most power disturbances are caused by the transfer of switches used to select power generation and transmission equipment for maintenance or reconfiguration following the failure of a component. The loss of operation experienced by most electronic equipment results from the depletion of internally stored energy following the power outage. This need not occur, for virtually all equipment can "ride through" very short losses of power. If the duration of the outage can be held to less than about 0.25 millisecond, the internal circuitry of most equipment will not be depleted and will continue to operate normally.
The power distribution system most common in the United States and aboard aircraft platforms is the threephase "wye" system shown in FIG. 1. The center of the three phases A, B and C is referred to as the neutral terminal and is typically connected in some manner to an earth or ground potential. The three phases are 120.degree. out of phase with each other and electrically rotate around the neutral terminal at a rate corresponding to the frequency of the power source. The neutral terminal may be used in conjunction with any one conductor to provide a source of single phase power, as shown in FIG. 2.
A number of different types of switching systems have been proposed for transferring multiphase loads between different multiphase A.C. power sources. In the simplest type of system, the load is disconnected simultaneously from all phases of the primary power supply and is then reconnected simultaneously to the corresponding phases of the backup or standby power supply. Because this changeover cannot be effected instantaneously, the load is left without power momentarily and disruptions of the type described above can occur. In a second type of system, which may be referred to as a "make before break" system, the load is momentarily connected in parallel between the two power sources as the transfer is taking place. This has the advantage of avoiding any temporary interruption of power to the load, but at the expense of possible damage to the power generation equipment or to sensitive loads as a result of differences in voltage, frequency or phase angle between the two power sources. The phenomenon of "motoring", in which one of two interconnected generators drives the other, can also occur in this situation.
Systems have also been proposed for transferring a load between two multiphase A.C. power sources on a phase-by-phase basis. In one known type of system, the detection of a trip condition in one phase of the primary power source causes that phase to be disconnected immediately from the load using transistor switching elements. The remaining phases may be disconnected at the same time as the first phase, or when the load current in each of the remaining phases reaches zero, depending on the particular manner in which the system is implemented. In either case, however, the system does not allow the phases of the backup power source to be connected to the load until the source voltage in each phase reaches zero. This is done in order to prevent current surges through the transistor switching elements, but it also has the effect of interrupting power to the load for an unnecessarily long period of time. This interruption can approach 1 millisecond in a 400 Hz power system, and can exceed 6 milliseconds in a 60 Hz power system. This time period is sufficiently lengthy to allow sensitive electronic equipment to fail due to the absence of power.
In a second known type of multiphase load transfer system, a matrix of antiparallel-connected silicon controlled rectifier (SCR) pairs is used to transfer the load between two power sources on a phase-by-phase basis. When a failure condition in the primary power source necessitates a transfer of the load to the secondary power source, the individual SCRs of the matrix are gated in such a manner as to momentarily connect each load phase in parallel between the primary source and the particular phase of the secondary source that matches the load phase voltage most closely. During this interval, the source phase having the higher voltage supplies power to the load. When the source phases commutate, the SCRs associated with the primary power source automatically turn off as the current passing through them reaches zero. This "make before break" switching procedure is advantageous in that it minimizes the interruption of power to the load, but it also requires that the individual SCRs of each antiparallel-connected SCR pair be controlled independently of one other in order to avoid loop currents from occurring between the two power sources. In addition, the dynamic matching procedure that is carried out between the phases of the load and those of the new power source introduces computational difficulties that are not encountered in systems which maintain the same phase relationship when transferring a load between different power sources.